iio:bmi160: use if (ret) instead of if (ret < 0)

We are using "if (ret < 0)" in many places in which the function returns 0
on success. Use "if (ret)" instead for better clarity and correctness.

Signed-off-by: Martin Kelly <martin@martingkelly.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>